A class-D amplifier is an electronic amplifier in which the amplifying devices (such as transistors) operate as electronic switches, instead of as linear gain devices as in other amplifiers. Generally, the signal to be amplified is a train of constant amplitude pulses, so the active devices switch rapidly back and forth between a fully conductive and nonconductive state. The analog signal to be amplified is converted to a series of binary waveform by pulse width modulation (PWM), pulse density modulation or other modulation before being applied to the amplifier. After amplification, the output pulse train is converted back to an analog signal by passing through a low pass filter. The class-D amplifier is more efficient than analog amplifiers because it reduces power waste as heat dissipation.
FIG. 1 shows a class D amplifier that includes a waveform generator 101, a comparator 102, a power amplifier circuit 103, a first filter circuit 104, a power supply 105 and a signal generator 106. The power supply 105 supplies operating voltage to the power amplifier circuit 103. The signal generator 106 generates a first input signal U1 and the waveform generator 101 outputs a second signal U2. The first and second input signals U1 and U2 are input to the comparator 102 which outputs a PWM signal U3. The PWM signal U3 goes through the power amplifier circuit 103 that outputs the amplified PWM signal U4. The amplified PWM signal U4 is input to the first filter 104 to obtain the audio output signal U0. The power of the output signal U0 depends on the duty cycle of amplified PWM signal U4 and the amplitude of the power supply 105 to the power amplification circuit 103. Thus, when the power supply 105 fluctuates, the audio output signal U0 may fluctuate even when input signals U1 and U2 remain the same. In short, the fluctuation of the power supply 105 causes audio distortion of the output signal U0. The users may hear the sound changes abruptly in that case. Thus, there is a need to a class D amplifier that can smooth the output signal U0.
One way to solve the above problem is to introduce a feedback circuit in the class D amplifier. As shown in FIG. 2, the feedback circuit 120 within the dashed box includes a filter circuit 122, a sampling circuit 124, and an integrator 126. The sampling circuit 124 takes the PWM signal U4 of the power amplifier 103 and outputs sampled PWM signal U5. The sampled PWM signal U5 is filtered by the filter circuit 122 to receive the signal U6, which passes through the integrator 126 and accumulates the input signal U1. The integrator 126 adjusts the amplitude of the input signal U1 according to the signal U6. Thus, the output of the waveform generator compares U2 with the adjusted input signal U2, thus making the output signal U3 and the signal U4 changes accordingly, which suppresses undesired change of the power U0. The feedback circuit 120, however, includes additional filter circuits after sampling. The feedback circuit 120 has to filter out the clutter and correct the phase offset of the analog signal, which leads to complex design of the feedback circuit. Further, the feedback circuit 120 introduces active device such as the integrator 126, resulting in an increase in circuit costs.
Thus, there is a need to a class D amplifier that can smooth the output signal U0 and reduces the complexity and cost of the feedback circuit.